"I think Cambridge Analytica is a major scandal which impacts on [the Brexit referendum]. There are very severe question marks about the conduct of the referendum and about the use of resources, and that is an emerging set of circumstances which throws many issues into question.

"I don't know where that will lead. But I think it would be foolish not to accept that this is a factor which I think is going to grow larger and larger."

Asked if it could prompt a second EU referendum, Russell said: "I think that's possible."

"You've got a situation where there are senior government figures who were allegedly aware of actions which would be contrary to electoral law. That is a very serious situation."

He also suggested there could be a criminal investigation into the matter.

Russell, an SNP veteran, has spearheaded the Scottish Government's attempts to keep Britain in the single market and avoid a hard Brexit which they say would be disastrous for Scotland's economy.

He said the emerging Cambridge Analytica scandal was one factor in a "heady political mix" which will threaten to derail negotiations with the EU. Others include the English local elections in May, where big losses could destabilise the Conservative party, the weak position of Prime Minister Theresa May, and the "inconsistent" position of the Labour party on Brexit.

The SNP has previously suggested that May's insistence on a 'hard Brexit' outside the customs union and single market could justify a second referendum on Scottish independence.

What is the Cambridge Analytica scandal?

This week a whistleblower, Christopher Wylie, suggested that data firm Cambridge Analytica and associated companies may have altered the outcome of both the U.S. presidential election and the Brexit referendum.

Wylie, the former director of research at Cambridge Analytica, told MPs this week that a Canadian business with ties to the firm's parent company, SCL Group, provided analysis for the Vote Leave campaign ahead of the EU referendum in June 2016. Wylie said that research likely breached the UK's strict electoral spending laws and may have swayed the vote, which was won by a margin of 52% to 48%.

A second whistleblower, Shahmir Sanni, claimed that Vote Leave donated £625,000 to the supposedly independent pro-Brexit campaign group BeLeave, which funnel led the money to a digital services firm with links to Cambridge Analytica, which would likely be a breach of spending laws.

Vote Leave's chief Dominic Cummings issued a forceful rebuttal of the claims on last week, describing the allegations as "factually wrong, hopelessly confused, or nonsensical" in a blog post.

He said stories alleging dodgy links with Vote Leave were the result of a "powerful set of people will do anything to try to shift public opinion in order that they can overturn the referendum."

He said the stories were being perpetrated by whistleblowers who were "peripheral, making invented claims about things they didn't see."